Electric-current transmission



R. C. MATHES ELECTRIC CURRENT TRANSMISSION Dec. 4, 1928. 1,693,955

Filed Aug. 22, 1923 2 Sheets-Sheet 2 Repeafer Delay Amplifier} m] Circuit Amplifier,

Rech'fier A32 Amplifier I Rectifier L /3/ Km] Delay Circuit Delay Repeafr Circuit, Amplifier I 230 I Amplifier A 1 i 'ri fier T Delay 1 Circuii, iJ/

hven/or: Rake/f C/Vq/hes.

Patented Dec; 4, 1928.

UNITED STATES PATENT amen ROBERT C. MATHES', OF WYOMING, NEW

JERSEY, ASSIGNOB TO WESTERN EIJ'EGTRIO COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

ELECTRIC-CURRENT 'rnalvsmssron.

Application iiled August 22, 1923. Serial No. 658,667.

This invention relates to the transmission of electric currents, and aims to provide improvements in the two way transmission such currents, for instance, in the two way repeating of telephonic currents.

. In the specific forms of the invention illustrated in the accompanying drawings, a repeater connected in a two wire circuit and comprising paths for transmitting in opposite directions has means associated with each path and responsive to transmission in that path for reducing thetransmission in the opposite direction, to thereby reduce echo effects in the circuit. The nature and effects of echo currents have been explained by A. B. Clark in his article on Telephone transmission over long cables, Journalof American Institute of Electrical .lngineers, January, 1923, page 1, and referencc may be understanding of the phenomena of echo ef.- fects. In the preferred form of the invention a delay circuit, that is, a circuit which has the propert of appreciably delayin the propagation o signalling currents t erethrough. is connected in one of the paths at a point beyond the point of association with that path of the means for reducing the transmission in the other, or second, path,

be operated in time to prevent currents which are'transmitted through the first path from flowing back through the second path. The second path is also equipped with .such a delay circuit. As explained hereinafter, the delay circuits increase the reliability of op eration of the echo suppressing means and reduce the time during which circulating currents can build up. The time required to render the desired repeating path inoperative is small because the necessary switching is accomplished by the mere opening of a contact; and therefore the amount of apparatus required in the delay circuit is small. 5 By a two wire line or circuit is meant a circuit of the type used where transmissions in both directions between two point geographically remote from each other are to be carried over the same pair of wires for the greater portion of the distance between the points, as distinguished from a four-wire cirhad to that article for a better" so that the transmissionreducing means will cuit which is a circuit of the type used where the transmission in one direction for the greater portion of the distance between two points geographically remote from each other is carried over a difierent pair of wires from the currents in the other direction.

Fig. 1 of the drawings is a circuit diagram of one form of the invention; and Fig. 2 is a similar showing of a form in which a delay circuit is employed and the switching is accomplished by the openin of a contact; and Figs. 3 and 4 show additional forms of the invention employing delay circuits and ac complishing the switching by the opening of a contact.

In Fig. 1 two-wire lines 5 and 6 forming part of a two-wire transmission system, for example, a telephone system, are. interconnected by a two-wire repeater comprising repeater amplifiers 7 and 8 for transmitting in the directions indicated by the arrowsand the usual three-winding transformers 9 and 10 and balancing networks 11 and 12. The amplifiers 7 and 8 may be of anysuitable type, but are preferably of the electron discharge type.

A relay 13 preferably of the polarized 'type has four windings fed by amplifierrectifiers 21, 22, 23 and 24 respectively, the amplifier-rectifiers 21 and 22 being fed from the hybrid coil or three winding transformer 9 and the amplifier-rectifiers 23 and 24 bein fed from the hybrid coil 10, as indicate so that for transmission east, that is,.for transmission from line 5 to line 6, amplifierrectifiers 21 and 24 receive current but, due to the property of conjugacy possessed by the hybrid coil connection, amplifier-rectifiers 22 and 23 receive little or no current, and, fortransmission west, 23 and 22 receive current but 24 and 21 receive little or no current. Each of the amplifier-rectifiers is shown merely as a block, and preferably comprises an electron discharge amplifying tube, or such tubes in tandem, feeding a three electrode electron'diseharge amplifying and rectifying tube. Forms of amplifier-rectifiers particularly suitable for operating voice relays are disclosed in R. V.- L. Hartley Patent transmitting electric energy, and -E. D.

so i

No. 1,588,186, issued June 8, 1926,

Jam Patent No. 1,526,550, issued Feb- 1 be of such forms.

7 passing r'uary 17, 1925, two-way transmission with repeaters, and my Patent No. 1,605,048, issued November 2,1926, all assigned to the assignee of this application, and the amplifier-rectifiers 21 to 24 may advantageously 1 The armature of relay 13 is normally biased to a neutral position between two contacts and 26, and the windings of the re lay tend to operate the armature about its pivot in the directions indicated by the ar rows on the respective windings. When talk is passing from west to east the relative ma itudes of the operating impulses are sac that the relay tongue (normally not in contact) is moved to contact 26, and repeater 8 then has its input short circuited, through a transformer 27. When talk is from eastto west the relay tongue s moved to contact 25, and repeater 7 then has its input short circuited, through a transormer 28. A' hybrid coil type of directional relay is claimed in my application Serial No. 653,624, filed July 25, 1923. Blocks N and N. represent networks for balancing the impedances of amplifier rectifiers 22 and as regards the hybrid coil 24, respectively,

not to disturb the impedance connectiomso as 1 balance of the circuit for the normal two we repeater function ofthis coil.

11 the operation of the system, when talk is passing from west to east, relay '13 closes its contact 26 as noted above, and consequently any echo currents or other currents which may tend to flow from line 6 into reeater-amplifier 8- or amplifier-rectifier 23 nov for instance to impedance unbalance at repeater stations in line 6 or reflection from terminal apparatus or the like, are short circuited, throu h transformer 27. Also, any current ten 'ng to enter the input of 8 or 23 from the output of 7, becauseof impedance unbalance between the line and network at hybrid coil 10, are short circuited,

through transformer 27. The re eration of the system in trans Knitting om east to west is similar to the i operation just described, and will be evident so i The lines 5 and G may, of course,

without further description.

I repeaters other than that shown connecting them, and any desired ones of such other sup:

repeaters-may be provided with echo pressing means of the characterdescribed above. Althou h where the number of re waters in tan cm; is large the number of possible echo paths islar e, the introduction of oneor two breaks or s ort circuits in the system me be made to cut down the numher of suc paths materially, and, furthermore, to out out those involving the greatest time lag.

The relay. 13, to be effective, must be embody h quicker operating than in the case of a voice relay for a four wire echo suppressor, such for instance as that disclosed in John Mills Patent 1,434,790, November 7, 1922, two-way transmision with repeaters, but should have an equivalent time of release, depending upon the type of circuit and the distance to the termlnal station, so that at least the primary echo current from that station will be prevented from passing the relay 13. If the release time is too long the relay will unduly interfere with the ability of the listener to break in on the talker.

The functioning of the circuit described above is such that when no speech is traversing the, circuit both repeaters are operative at substantially their full gain mitted by the degree of balance obtained between the connected lines and their balancing networks, but when speech is applied the inactive repeater path is rendered inoperative to transmit. This enables the repeaters to be set much nearer their singing points. Further, if desired the relay 13 may be made to effect an input potentiometer adjustment or-otherwise increase the gain of the active repeater when decreasing the transmitting efliciency of the inactive repeater, after the general fashion set forth hereinafter in connection-with the description of Fig. 2.

If desired, the amplifier, rectifiers 21 and 23 and the switch windingsfed by them, or the amplifier-rectifiers 22. and 24"and the switch windings fed by them, may be omitted from the system of Fig. 1. It should be noted that the input to amplifier-rectifier In Fi 2 the elements 5'. to 12 correspond to the e cments 5 to 12, respectively, of Fig. 1. Delay circuits and 31 are preferably onnected in the repeater paths comprising the amplifiers 7 and 8, respectively. These delay circuits are preferably filters connected in the output circuits of the repeater-ampli- 'ers, and, as repeater filters, function in the manner disclosed in G. A. Campbell Patent 1,227 ,114, May 22, 1917, electrical receiving, translating, or repeating circuit. The filters may be, for example, low pass filters such as those shown at 1 in Fig. 11 of the patent, or as shown in Fig. 7 of the patent, comprising recurrent sections each having a series arm containing lumped inductance and a shunt arm containing lumped capacity. The filters are here employed not merely for the purpose of frequency separation, but especially as delay circuit to delay the propagation of signaling currents therethrough for omitted they should-preferably be 21 and reasons which will be made apparent hereinafter. The delay circuits need not necessarily have filtering properties but may be circuits of any type suitable for producing the desired delay in current propagation.

An amplifier rectifier 32 such as is referred to above has its input connected, for example, between the output of repeater element 7 and the input of delay circuit 30, and feeds the left hand winding of a relay 33, the left hand winding of a 'relay 34, and

the winding of a relay 45, the other windings of the relays 33 and 34 and the winding of a relay 46 being fed from a similar amplifierrectilier 35 connected to the transmission path including repeater element 8 and delay circuit- 31 as the amplifier-rectifier 32 is connected to the oppositely directed repeater path; The relay 33 is normally biased to its left hand contact. and the relay 34 is normally biased to its right hand contact. The windings of these relays tend to operate the relay armatures in the directions indicarted-by the arrows on the respective windmgs.

The transmission path between the hybrid coil 9 and the input of repeater element 7 extends through the armature and left hand contact of relay 33 and the armatureof a normally closed relay 36 in series, and when relay 33 moves its armature to its right hand contact, a local circuit through the winding of relay 36 is closed, to cause relay 36 to open its contact. When relay 33 thereafter again closes its left hand contact, the transmission path between hybrid coil 9 and the input of repeater element 7. still remains open until relay 36 closes. Relay 33 is preferably designed to close its right hand contact quickly, and relay 36 is preferably designed to be slow in releasing its armature, for reasons which will be made apparent hereinafter. v

The relay 34, with the cooperation of a relay 38 corresponding .to relay 36, controls the repeater path west in the manner in which the relay 33 withthe cooperation of relay 36 have just been described as controlling the repeater path east. It is not essential that this control of these paths be at the input sides of the repeater elements in the respective paths.

A unilaterally transmitting device 40, such for instance as a three electrode electron discharge amplifier, may be inserted between the input of amplifier-rectifier 32 and the hybrid coil 10, to prevent transmission which approaches the. repeater over line 6 from causing energization of the relay windings fed by that amplifier-rectifier. A similar unilateral device 41 is likewise inserted in the repeater path west. As will be explained hereinafter the devices 40 and 41 on the one hand, and the left hand winding of relay 33 and the right hand winding of relay omitted if devices 40 and 41 are employed.

The relay 45, when energized, opens a shunt containing a resistance 47 connected, for example, across the input of the amplifier 40, to thereby raise the gain of the eastwardly directed repeating path between lines 5 I and .6 by any desired amount Similarly, the relay 46, when energized, opens a shunt circuit containing a resistance 48. connected across the westwardly directed repeating path, to thereby raise the gain of that path by any desired amount. The values of resistances 47 and 48 are such that, when the system is not in use'the total transmission gain around the loop comprising the oppositely directed repeating paths is less than the; total transmission loss around that loop, or in other words the transmission equivalent of the circuit around the loop is greater than zero, by an amount sufficient to prevent singing of the repeater due to noise currents on the line. The relay 45 should be designed to open at the same time as the right hand contact of relay 34, or

slightly later, so that the relay 45 will be open when transmission first emerges from delay circuit 30, in order that the initial 'part of the transmission-may be repeated at the full gain of the repeater with shunt 47 removed, and yet the repeater may not sing due to increase of the gain in the eastwardly directed repeating path before the ,gain of the westwardly directed repeating path has been lowered by the opening of the path at the right hand contact of'relay 34; and the relay 45 should be so designed that its time of closing is slightly greater than the time of delay of transmission by the delay'circuit 30. so that the gain of the eastwardly directed repeating path will not-be reduced by the closing of shunt 47 until the eastwardly directed transmission has ceased to enter line 6. The relay 46 should be designed to open at the same time as the left hand contact of relay 33, or slightly later, and its closing time should be slightly greater than the time of delay of transmission by delay circuit 31. c

For conditions of operation in which the time factors involved do not call for very high speed switching, as will be discussed more fully below, vibrating alternating-con rent relays, for instance of the type disclosed in S. G. Brown Patent 1,185,472, May 30, 1916, may be used instead of the amplifier-rectifiers 32 and 35 and relays 33 and 3'4 to open the repeater paths.

The operation of the system will now be described for transmission from West to east and the operation for transmission from east to west, which is similar, will then be e'vident without further description.

the output of 7 is taken off by amplifierrectifier 32 and applied to relays 33, 34, and 45 to hold the armature of relay 33 more firmly on its left hand contact and to move the armature of relay 34 from its right hand to its left hand contact, thereby opening the input of repeater element 8.

If the unilateral device 41 be omitted from circuit, the part of the energy from line 5 which enters the series winding of hybrid coil 9' will get through delay filter 31 to the input of amplifier-rectifier 35 andtend to oppose the action of the output of amplifierrectifier 32. However, this part of the energy from line 5 is less than the energy fed to the input of amplifier'rectifier 32 by repeater-amplifier 7', and it acts later by the time of transmission through delay circuit 31. It is therefore improbable that the initial operation of relays 33 and 34 will be the reverse of that desired. 'Any possibility there might be of such undesired action can beeliminated by the insertion of the unilateral device 41 in the circuit. When the device 41 is employed, the left hand winding of relay 33 may well be omitted. Similarly, when the device is employed the right hand winding of relay 34 may well be omitted.

As soon as relay 34 opens, all circulating unbalance currents are sto ped and echoes from line 6 are cut off. s soon as relay 45 opens, the gain of the eastwardly directed repeating path between line 5 and line 6 is increased by any desired amount; and this will not cause the repeater to sing, since the loss in the oppositely directed repeating path between lines 6 and 5 has been increased y a greater amount at the right hand open contact of relay 34. Thus, by employing the relays 45 and 46 and the gain controlling means 47 and 48 thelimit of the permissible gain of the, repeater whilein use is raised from the value just within a working margin of the singing point when the repeater is not in use to any desired value. This feature is of special utility in cases where good balance between the lines 5' and 6' and their balancing networks 11 and 12 can only'be attained with unwarranted expense, due for instance to impedance irregularities in the lines.

Upon cessation of talk. the closing of the input to repeater-amplifier 8 should be delayed by a'tinie equal to the time of the longest echo, and this is accomplished by the relay 38. t

The seriousness of the effect of the circulatin currents which are set up before the switc 1mg operation takes place depends upon the length of time of the switching and also upon how great an increase of gain maybe desired from the repeater when provided with the echo suppressing means, that is, also upon how close it is desired to make the gain of the repeater approach its singing point for the condition of the circuit in which both the repeating path east and the repeating path west are in condition to transmit. The circuit of Fig. 2 gives a very low switching time since the switching operation is performed by merely opening a contact and the time required for opening a contact is far less than the time required for the travel of an armature to a satisfactory make or closing contact. Furthermore, the amount of circulating current builtup may be reduced to any desired degree by increasing the time lag introduced by the delay filters, for instance, by increasing the number of sections in the filters; for if the transmission in one of the repeater paths is delayed in the filter in that path until the oppositely directed path has been opened by the echo suppressing switch, no circulating current can flow.

When the time lag in the filters equals the switching time even the initial unbalancecurrents, which tend to cause distortion of the initial part of the transmission and false operation of the switch controlling the active repeater path and which result from the fact that the transient impedance of a network such as 12' differs-from that of a perfectly regular line such as 6 for instance would be in the ideal case, have no opportunity to circulate. Under those circumstances the gain of the repeater amplifying elements may be pushed near the singing point without injuring the quality of transmitted speech. On the other hand, if it is only, necessary to eliminate the longer echoes in a circuit without giving it a lower transmission equivalent or loss, the time of switching need not be so short or the lag in the delay circuits so great. In such a circuit the gain of the repeater is usually so far below the singing point that the local circulating currents do not seriously affect the quality. The actual requirements, as regards switching time and time of propagation through the delay circuit, may lie all the way between these two extremes. The use of a guick operating switching means tends to re uce the number of sections of delay input-to the amplifier-rectifier, 32, so that when amplifier-rectifier 35 has caused relay 33 to open the repeating path east any current tending to flow from hybrid coil 9 into the repeating path east, such for instance, ascurrent due tounbalance between line 5 and network 11', will be-prevented from entering the input of amplifier-recth fier 32 and tendin to cause false operation of relay 34. Similar considerations of course apply to the matter of the location, in the repeater path west, of thebreak introcluced by relay 34.

The application of a delay circuit'to a voice controlled repeater to delay transmission until tlie voice operated switching means has had time to operate is claimed in H. D. Arnold application Serial No. 338,759,

November 17, 1919, two-way repeater circuits, assigned to the assignee of this application.

It has been intimated above that, in Fig. 1, if either the amplifier rectifiers 21 and 23 or the amplifier rectifiers 22 and 24 are to be omitted, the ones omitted may preferably be 21 and 23. However, this ordinarily does not hold if, in Fig. 1, it is desired to insert delay circuits in, for instance,'- the outputs of repeater amplifiers 7 and 8 for it is then advantageous to have the delay circuit in the outputof 7 delay eastwardly directed transmission until "21 has caused the' operation of 13, and to have the other delay circuit delay westwardly directed transmission until 23 has caused operation 0f13.'

In Fig. 3ythe elements 105 to 112 correspond tot-he elements 5 to 12, respectively,

of Fig.2, and the elements 130, 131', 132, 135, 136 and 138 correspond to elements 30, 31, 32, 35, 36 and 38, res ectively, of Fig. 2. Also, the relay 133 of Fig. 3 corresponds to the relay 33 of Fig. 2, except that the relafy 133 has no winding corresponding to the le "hand winding of relay33. Similarly relay 134 of Fig. 3 corresponds to the relay 34 of Fig. 2 with its right hand winding omitted.

It will be noted that the devices 40 and 41 of Fig. 2 are also absent in Fig. 3, .for, as

from the description above ofthe operation of Fig. 2. In transmitting from west to cast in Fig. 3, a part of the energy coming to the repeater from line 105 is transmitted through the hybrid coil 109 and amplified by the repeater element 107 in the usual fashion, and istransmitted throu h the armatures of relays 133 and 136; the clay circuit 130,

gap in 234 and 233 by and theprimary winding of the input transformer for amplifier rectifier .132, to the hybrid coil 110, where it divides between the line 106 and the network 112 as usual. The secondary winding of the input transformer for amplifier rectifier 132 therefore feeds elk orgy to the amplifier rectifier'132, and consequently relay 134 opens the westwardly directed repeating path in which the primary winding of the input transformer for amplifier rectifier 135 is serially included, and closes the circuit of'the slow releasexrelay 138 which thereupon establishes a" second h the westwardly directed repeating hat. g 1

As soon as relay-134 opens its right hand contact, not only are all circulatingfunbal ance currents due to impedance unbalance between line 106 and network112 stopped,-

and eclioesfrom line 106 cut off, but, further, the part of the energy from line' 105 which enters the series winding of the hy brid coil 109 and would tend-topass through the delay circuit 131,'thepr,in'1'ary winding of the input transformer for amplifier rectifier 135 and the output of'repeater amplifier 108 and cause false operation of relay 133, is prevented from flowing through the primary winding of the input transformer-for amplifier rectifier 135; andsince the delay circuit 131 prevents such-energywestwardly directed repeater path from hybrid coil110, is prevented.

Upon cessation of the transmission east the relay 138 delays the closure of the westwardl directed repeater path and the inut o amplifier rectifier 135, the relay 138- eing slow releasing as is the relay 38 of Fig. I 1 In transmitting from line 106 toline 105, the operation of the system is similar to the operation just described for transmissionfrom line to line 106, andneed not be further described.

In Fig. 4 the elements 205 to 238 corre.- spond to the elements 105 to 138, respectively, of Fig. 3, but instead of havin' the inputs of amplifier rectifiers'232 an 235 connected in series in the repeating path east and the repeating path west, res ectively, to avoid false operation .ofthe re ays currents from the series windings of hy object is accomplished by locating the repeater amplifiers 207 and 208 in the repeatid coils 210 and 209, this ing paths at points beyond, instead of in advance of, the points of association of the inputs of the amplifier rectifiers with these paths. That is, the location of the repeater amplifiers 207 and 208 in Fig. 4 is similar to the location of the unilateral devices and 41 in Fig. 2, so that the repeater amplifiers 207and 208 of Fig. 4 serve not only as repeater amplifiers but also'accomplish the function which the devices 40 and 4] accomlplish in Fig.2.

T us, in the operation of this system for transmittin fromline 20.5 to line 206 'repeater amp ifier 208 prevents current from the series winding of h brid coil 209 from entering amplifier recti er 235 and causing false operation of relay 233. Similarly, in transmitting from line 206 to line 205 the repeater amplifier 207 prevents current from the serles winding of hybrid coil 210 from entering amplifier rectifier 232 and causing false-operation of relay 234. With this explanation the o eration of the system of Fig. 4 will be evi ent from the descriptions, above, of the operation of the systems of Figs. 2 and 3.

It is clear that, if desired, the relays 133 and 1340f Fig. 3 and the relays 233 and 234 of Fig. 4 may be provided with windings corresponding to the left hand winding of relay 33 and right hand winding of relay 34 in Fig.2; and that if desired, in Fi 3 unilateral devices such as 40 and 41 of ig. 2 ma be inserted, for instance, between delay circuit 130 and hybrid coil 110 and between delay circuit 131 and hybrid coil 109,

respectively; and that, if desired, the systems of Figs. 3 and 4 maybe provided with shunts across the outputs of the delay circuits, and relays for opening the shunts, as the system of Fig. 2 is provided with the shunts 47 and 48 across the outputs of delay circuits 30 and 31 and relays and 46 for opening those shunts. I

The general principles herein disclosed ma be embodied in many organizations widizly different from those specifically illustrated and described without de arting from the s irit of the invention de nedin the a pen ed claims. 1

Alt ough the invention has been set forth with especial reference to its applicationto the repeating of voice frequency currents, it is of course also applicable to the transmission and repeating of currents of other frequencies, whether hi her or lower, as for instance, currents of t e frequencies com- ,monly used for radio and carrier currents.

What is claimed is: 1. A transmission system COIDPIlSlIlg two two-wire circuits, two unilateral paths each including an amplifier for transmitting signal currents in op osite directions between said circuits, a hybrid coil connection between said unilateral paths and each of said twowire circuits, said unilateral paths being operative to transmit in the absence o1 transmission in the system, and means for preventing cluring transmision in the systemobjectionable echoefiects due to the return currents transmitted through the hybrid coil connections from one unilateral path to the a other, said means comprislng means connected to one unilateral path and actuated. in response to transmission of signalin currents thereinto disable the other unilateral path and a delay circuit connected in one of said unilateral paths between the point of connection of the disabling means to. said one path and'the disabling point in said other path for delaying the passage of signaling currents in said unilateral paths for a time interval sufiicient to prevent said 'return currents from reaching the disabling point in said other path before the disabling means has operated.

2. A system in accordance with that of 4. A- repeating system comprising two two-wire circuits, two opcpositely directed unilateral paths each inclu ing an amplifier, a hybrid coilconnection associating each twowire circuit with the input of one unilateral path and the output of the otherunilateral path, each of said paths being conditioned to repeat signaling currents between said twowire circuits in the absence of transmission in said system, the transmission equivalent around the 100 comprising said paths then being greater t an zero, and means effective during. transmission in the stem for preventing objectionable echo ei fiects due to the return currents transmitted through the hy-' brid coil connections from one unilateral path to the other, said means comprising means connected to one unilateral .path and actuated inresponse to transmission of signaling currentstherein to disable the other unilateral path and means inserted in in said unilateral paths for delaying the passage of said signaling currents therein for a time interval sufficient to prevent said return currents from reaching the disabling'point in said other has 0 'erat 5. two-way transmission system comprising two two-wire circuits, two unilateral path before the disabling means paths for transmitting in opposite directions between said circuits, a movable con tact for controlling transmission in one dimotion in sand system, means comprising an eeann two points close together in said path untilsaid contact has operated.

6. A two-way transmission system comprising two two-wire circuits, two unllateral paths for transmitting in opposite 'dir ections between said circuits, means responsive to transmission in one direction inthe system for opening said path in the other direction without delay due to'other mechanical motion, said means comprising. an electro--' magnetic relay having a contactin said path in said other direction and an electron discharge amplifier rectifier for operating said relay in response to said transmission in said one direction, and means connected in said path in said one direction for delaying transmission between, two, points close together in said path until said electromagnetic relay has opened said contact. I

7 A 22-type repeater comprising two oppositely directed repeating paths, a relay operable to reduce the transmitting efliciency of the path in one of said directions, means for operating said relay responsive to transmission in the other direction, and means comprising a movable contact for maintaining said reduction in transmitting efliciency during a time interval following the restora tion of said relay.

8. A transmission system comprising two two-wire circuits, two unilateral paths for transmitting in opposite directions between said circuits, means, comprising a relay, con-' nected to one path and responsive to transmission inthat path for rendering the other path inoperative, and means connected to said other path and responsive to transmission in said other path-and comprising a winding on said relay for preventing operation of said relay in response to transmission in said one path.

9. A 22-type repeater comprising two oppositely directed repeating paths, eachof said paths including a re eater amplifier,

means connected to eachpat and responsive to transmission in the connected path for rendering the oppositely directed path inoperative, and a unilaterally transmitting means connected in one ath for excluding from transmission to said means for rendering said other path inoperative, the energy approaching said repeater in the direction in which said other path transmits.

10. A two-wire transmission line and a repeater connecting two sections thereof, said repeater comprising paths operative to transmit in opposite directions between said sections in the absence of transmission in said system, and means associated with said paths, and responsive to a given transmission in circuits, one 0 directed repeating paths and comprising echo-suppressing means, said means comprising a relay-operable to IGdIlCQQthBgtIfillS- mitting efliciency of the path in one ofsaid directions, means for operating saidrelay ,75

responsive to transmission in the. othendi'rection in said system, andmeanscomprisinga movable contact for maintaining saidreduction in transmittin eificiency after. restora-v tion of said relay, or atime approximately '86 equal to the interval, between the pas'sa e through the repeater and the returni-totfie repeater of the current representing the long- 'estecho that is to 'be suppressed. p I Y t 12. A two-wiretransmission line and a repeater connecting two sections thereof, said repeater comprising paths operative to trans-. mit in opposite directions between said sections in the absence of transmission in. said system, means tendingto prevent, currents so, from one of said paths from flowing in the other of said paths, and means including two rectifiers, each responsive to transmission in one direction in the system for substantially preventing the passage of current 05 through the path in the other direction and .two-wire circuits, two unilateral paths con-' ditioned to repeat in opposite directions between 'said circuits in the absence of, transmission in the system, the transmission equivalent around the loop comprising said paths being then greater than zero, means operable .for controlling transmissionin said system, and means inserted in said paths for delayin transmission thereth'rough substantial yuntil said transmission controlling means has operated.

14. A re eating system comprisin two-1 two-wire clrcuits, two unilateral pat s for repeating in opposite directions between said said paths bein in condition to repeat between sa d circuits in the, absence of transmission in said system but the tr ms-- mission equivalent aroundithe' loop comprising-said paths being thengreater than zero,

means operable for controlling transmission v insaid system, and means inserted-in said paths for. delaying transmission therethrough substantially until said transmission controlling means has operated.

15. A transmission system comprising a two-wire line and a repeater connecting two sections of said line, said repeater comprising paths operative to transmit in opposite directions between said sections in the absence of transmission in said system, means tending to prevent currents from one of said paths from flowing in the other of said paths, rectifying means responsive to transmission-in the system in the direction in which said one path transmits, rectifying means responsive to transmission inthe system in the opposite direction, means controlled by said two transmission responsive means for substantially preventing passage of current through said other path and through the second mentioned transmission responsive means during said transmission in the system in the direction in which said one path transmits, and for substantially preventing passage of cur-rent through said one path and, through the first mentioned transmission responsive means during said transmission in said opposite direction.

1 6. In a transmission system, two unidirectional'repeating paths for repeating in opposite directions, and echo suppressing means, said means comprising a relay operable to reduce, the transmitting efliciency in one of said directions, means for operating said relay upon flowof transmission in the other direction in said system, and a relay comprising a movable contact for maintaining said reduction in transmitting efficiency after restoration of said first relay and causing said reduction to continue in effect. after initiation of said restoration, for a time ap proximately equal to the interval between the arrival at the echo su pressor and the return to the suppressor o the current representing the echo that is to be suppressed.

17. A transmission system comprising a circuit for transmitting in opposite directions, two unidirectional, oppositely directed repeating paths for transmitting to and from said circuit, respectively, means operable to reduce the transmitting efiiciency of the path in one direction, means for operating said first-means upon flow of transmission in the other direction, and means comprising a movable contact maintaining said reduction in transmitting efiiciency in effect during a time interval following initiation of restoration of said first means to its unoperated condition.

In witness whereof, I hereunto subscribemy name this 20th day of August, A, D.,

ROBERT C; MATHES. 

